Reporting back, both repositories rustc2duchain and kdev-rust don't compile or recent ArchLinux. Incompatible versions of kde and rustc. As I have no idea about KDESDK and no time to investigate it, I find it unusable for now.

  CMake Error at CMakeLists.txt:22 (find_package):
  Could not find a configuration file for package "KDevPlatform" that is
  compatible with requested version "4.90.91".

  The following configuration files were considered but not accepted:

    /usr/lib64/cmake/kdevplatform/KDevPlatformConfig.cmake, version: 1.7.3
    /usr/lib/cmake/kdevplatform/KDevPlatformConfig.cmake, version: 1.7.3
    /lib64/cmake/kdevplatform/KDevPlatformConfig.cmake, version: 1.7.3
    /lib/cmake/kdevplatform/KDevPlatformConfig.cmake, version: 1.7.3